This invention relates to the dehydration of organic oxygenates such as alcohols. More particularly it relates to a membrane technique for effecting separation of water from an aqueous mixture containing alcohols such as isopropyl alcohol.
As well known to those skilled in the art, it is possible to remove water from mixtures thereof with organic liquids by various techniques including adsorption or distillation. These conventional processes, particularly distillation, are however, characterized by high capital cost. In the case of distillation for example the process requires expensive distillation towers, heaters, heat exchangers (reboilers, condensers, etc.), together with a substantial amount of auxiliary equipment typified by pumps, collection vessels, vacuum generating equipment, etc.
Such operations are characterized by high operating costs principally costs of heating and cooling--plus pumping, etc.
Furthermore the properties of the materials being separated, as is evidenced by the distillation curves, may be such that a large number of plates may be required, etc. When the material forms an azeotrope with water, additional problems may be present which for example, may require that separation be effected in a series of steps (e.g. as in two towers) or by addition of extraneous materials to the system.
There are also comparable problems which are unique to adsorption systems.
It has been found to be possible to utilize membrane systems to separate mixtures of miscible liquids by pervaporation. In this process, the charge liquid is brought into contact with a membrane; and one component of the charge liquid preferentially permeates the membrane. The permeate is then removed as a vapor from the downstream side of the membrane--typically by reducing the pressure below the vapor pressure of the permeating species or by sweeping with a carrier gas.
Illustrative membranes which have been employed in prior art separation techniques include those set forth in the following table:
______________________________________ Polyacrylonitrile Neel, Aptel & Clement Desalination 53, 297, (1985) Crosslinked Eur. Patent 0 096 Polyvinyl Alcohol 339 to GFT as assignee of Bruschke ______________________________________
The cost effectiveness of a membrane is determined by the selectivity and productivity. Of the membranes commercially available, an illustrative membrane of high performance is that disclosed in European patent 0 096 339 A2 of GFT as assignee of Bruschke - published 21 Dec. 1983.
European Patent 0 096 339 A2 to GFT as assignee of Bruschke discloses, as cross-linking agents, diacids (typified by maleic acid or fumaric acid); dihalogen compounds (typified by dichloroacetone or 1,3-dichloroisopropanol); aldehydes, including dialdehydes, typified by formaldehyde. These membranes are said to be particularly effective for dehydration of aqueous solutions of ethanol or isopropanol.
This reference discloses separation of water from alcohols, ethers, ketones, aldehydes, or acids by use of composite membranes. Specifically the composite includes (i) a backing typically about 120 microns in thickness, on which is positioned (ii) a microporous support layer of a polysulfone or a polyacrylonitrile of about 50 microns thickness, on which is positioned (iii) a separating layer of cross-linked polyvinyl alcohol about 2 microns in thickness.
Polyvinyl alcohol may be cross-linked by use of difunctional agents which react with the hydroxyl group of the polyvinyl alcohol. Typical cross-linking agent may include dialdehydes (which yield acetal linkages), diacids or diacid halides (which yield ester linkages), dihalogen compounds or epichlorhydrin (which yield ether linkages) olefinic aldehydes (which yield ether/acetal linkages), boric acid (which yields boric ester linkages), sulfonamidoaldehydes, etc.
See also J. G. Prichard, Polyvinyl Alcohol, Basic Properties and Uses, Gordon and Breach Science Publishers, New York (1970) or
C. A. Finch, Polyvinyl Alcohol, Properties and Applications, John Wiley and Sons, New York (1973).
Also of interest is U.S. Pat. No. 4,755,299 to GFT Ingenieureburo fur Industrieanlagenbau as assignee of H. Bruschke directed to a separation process using e.g. polysulfone bearing polyvinyl alcohol separating layer which has been cross-linked (e.g. col 2, line 66 to column 3, line 7 inter alia) by esterification with dicarboxylic acids, etherification with acids or dihalogen compounds, or acetalization by means of aldehydes or dialdehydes (note e.g. formaldehyde Ex 3); and European Specification 0 096 339 published 21.12.83 Bulletin 83/51 to the same assignee by the same inventor.
U.S. Pat. No. 4,802,988 which issued Feb. 7, 1989 to Texaco as assignee of John Reale, Jr. and Craig R. Bartels is directed to separation of aqueous organic oxygenates by pervaporation with a membrane of polyvinyl alcohol which has been cross-linked with aliphatic polyaldehydes having .gtoreq.3 carbon atoms.
U.S. Pat. No. 4,941,976 which issued Jul. 17, 1991 to Texaco as assignee of Craig R. Bartels and John Reale, Jr. (a divisional of U.S. Pat. No. 4,802,988) is directed to the membrane.
U.S. Pat. No. 5,004,861 which issued Apr. 2, 1991 to Texaco as assignee of Mordechai Pasternak, Craig R. Bartels, and John Reale, Jr. is directed to separation of water from a charge mixture of hydrocarbon/organic oxygenate by pervaporation through a membrane of (i) polyvinyl alcohol which has been cross-linked with aliphatic polyaldehydes having .gtoreq.3 carbon atoms.
U.S. Pat. No. 4,961,855 which issued Oct. 9, 1990 to Texaco as assignee of Craig R. Bartels and John Reale, is directed to separation of water/organic oxygenates (such as isopropanol) by pervaporation through a membrane of polyvinyl alcohol which has been cross-linked with aliphatic polyaldehydes having .gtoreq.3 carbon atoms.
U.S. Pat. No. 4,935,144 which issued Jun. 19, 1990 to Texaco as assignee of Mordechai Pasternak, Craig R. Bartels, and John Reale, Jr. is directed to separation of ketones from aqueous systems by pervaporation through a membrane of polyvinyl alcohol which has been cross-linked with aliphatic polyaldehydes having .gtoreq.3 carbon atoms.
U.S. Pat. No. 4,910,344 which issued May 20, 1990 to Texaco as assignee of Mordechai Pasternak, Craig R. Bartels, and John Reale, Jr. is directed to separation of organic oxygenates from aqueous systems by pervaporation through a membrane of polyvinyl alcohol/polyacrylic acid.
U.S. Pat. No. 4,960,519 which issued Oct. 20, 1990 to Texaco as assignee of Mordechai Pasternak, Craig R. Bartels, John Reale, Jr. and Vatsal M. Shah is directed to separation of alcohol from organic oxygenate by pervaporation through a membrane of polyvinyl alcohol/polyacrylic acid.
U.S. Pat. No. 4,971,699 which issued Nov. 20, 1990 to Texaco as assignee of Craig R. Bartels is directed to separation of water/organic oxygenate by pervaporation through a membrane of polyvinyl alcohol/polyacrylic acid.
It is an object of this invention to provide a novel composite membrane characterized by its ability to effect separation of water from organic oxygenates such as isopropyl alcohol. Other objects will be apparent to those skilled in the art.